I had to teach a class on Friday and it involved NHD Data.
NHD data is supposed to be an ever evolving dataset. The beginning's of
it are 1:24k USGS Topographic Maps. As time goes on and Lidar
<http://en.wikipedia.org/wiki/LIDAR>becomes more prevalent the dataset
will improve. Tennessee is slated to get a high resolution dataset of
NHD data collected form photogrametrically acquired data in the next few
years.
Because NHD data is based of 1:24k quad sheets (and in come cases USFWS
Wetland Maps) it's dated - in Chattanooga it's probably 40 to 50 years
old. Streams change. Ponds disappear. Things become channelized. If you
compare it to the NAIP or Bing Aerial Imagery in some cases it's
remarkably close and in some it so far off you wonder what happened.
There is also a second glitch with the data - since NHD is based off the
1:24k topo maps it's not entirely accurate. The USGS changed their
definitions of what consisted of a blue line stream from "it's a drain"
to "it's got water in it". It didn't affect Lakes/Rivers so much but the
blue line streams are questionable unless they are viewed with Aerial
Photography (and in my opinion need to be viewed in Stereo or site visit
to see what is occurring with it).
I say all of that - it's better than having nothing. At least here we
can improve it.
Randy
Randal Hale, GISP
North River Geographic Systems, Inc
http://www.northrivergeographic.com
423.653.3611 [email protected]
twitter:rjhale
http://about.me/rjhale
On 4/9/2012 2:11 AM, Martijn van Exel wrote:
On 4/9/2012 12:00 AM, Clifford Snow wrote:
On Sun, Apr 8, 2012 at 10:39 PM, Martijn van Exel <[email protected]
<mailto:[email protected]>> wrote:
Hi,
Remapping in CA, I come across some weird stuff.
Here's some NHD 'data':
http://www.openstreetmap.org/?__lat=35.17764&lon=-119.12641&__zoom=16
<http://www.openstreetmap.org/?lat=35.17764&lon=-119.12641&zoom=16>
Either the aerial imagery is way off here, or this is just bad data.
If it is, I presume that there has been a review of this data before
import, this is the exception, and the vast majority of imported NHD
objects actually do represent reality. I hope.
--
Martijn van Exel
Are you talking about the water - lakes and ponds? From reading
nmixter's diary, he/she has posted comments about mapping farms. One
comment suggested taking the import to the talk-us mailing list. BTW - I
did just drive through some farm land in Western Washington. Farmers
had dug temporary canals to help drain (or so I assumed) the water from
the field so they could plant. I probably wouldn't map it unless they
were permanent.
Yes, I see a lot of water features that are just not corroborated by
the aerial imagery, which could mean one of at least three things:
1) The aerial imagery is out of date
2) The NHD data is out of date
3) The NHD data represents something I don't understand (the future, a
temporary situation (which should not be in OSM), something underground?)
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